Method for preparing microparticles having a selected polymer molecular weight

ABSTRACT

A method for preparing microparticles having a selected polymer molecular weight. The hold time and temperature of a solution containing a nucleophilic compound and a polymer having a starting molecular weight are controlled in order to control the molecular weight of the polymer in the finished microparticle product. In this manner, a selected polymer molecular weight in the finished microparticle product can be achieved from a variety of starting material molecular weights.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to preparation of microparticles.More particularly, the present invention relates to a method and anapparatus for preparing microparticles having a selected polymermolecular weight.

[0003] 2. Related Art

[0004] A variety of methods is known by which compounds can beencapsulated in the form of microparticles. It is particularlyadvantageous to encapsulate a biologically active or pharmaceuticallyactive agent within a biocompatible, biodegradable wall forming material(e.g., a polymer) to provide sustained or delayed release of drugs orother active agents. In these methods, the material to be encapsulated(drugs or other active agents) is generally dissolved, dispersed, oremulsified, using stirrers, agitators, or other dynamic mixingtechniques, in one or more solvents containing the wall formingmaterial. Solvent is then removed from the microparticles and thereafterthe microparticle product is obtained.

[0005] One variable that affects the in vitro and in vivo performance ofthe microparticle product is the molecular weight of the polymer orpolymeric matrix material in the final microparticle product. Molecularweight affects drug release characteristics. The molecular weight of apolymer influences the biodegradation rate of the polymer. For adiffusional mechanism of active agent release, the polymer should remainintact until all of the active agent is released from themicroparticles, and then degrade. The active agent can also be releasedfrom the microparticles as the polymeric matrix material bioerodes. Byan appropriate selection of polymeric materials a microparticleformulation can be made in which the resulting microparticles exhibitboth diffusional release and biodegradation release properties. This isuseful in affording multiphasic release patterns.

[0006] It has been reported that the molecular weight of thepoly(D,L-lactide) (“DL-PL”) component of microcapsules containing up to50% thioridazine free base decreased during fabrication, and indissolution rate studies of the microcapsule (see Maulding, H. V. etal., Biodegradable Microcapsules: “Acceleration of Polymeric ExcipientHydrolytic Rate by Incorporation of a Basic Medicament”, Journal ofControlled Release, Volume 3, 1986, pages 103-117; hereinafter “theMaulding article”). The results reported in the Maulding article revealthat the degradation rate of DL-PL in ketotifen free base microcapsuleswas greater when the encapsulation process was carried out at 4° C. thanit was when the encapsulation process was carried out at 25° C. Incontrast, the degradation rate of DL-PL in thioridazine free basemicrocapsules was greater when the encapsulation process was carried outat 23° C. than it was when the encapsulation process is carried out at4° C. Based on these results, the Maulding article suggestscircumventing the polymer degradation by carrying out the preparation ofmicrocapsules at 4° C. in the case of thioridazine base. The Mauldingarticle does not provide a method by which the molecular weight of thepolymer in the finished microparticle can be conveniently controlled.Nor does the Maulding article provide a method for preparingmicroparticles that have a selected polymer molecular weight in thefinished microparticle product.

[0007] Thus, there is a need in the art for an improved method forpreparing microparticles that controls the molecular weight of thepolymer or polymeric matrix material in the finished microparticleproduct. There is a particular need in the art for an improved processthat provides a method for preparing microparticles that have a selectedpolymer molecular weight. The present invention, the description ofwhich is fully set forth below, solves the need in the art for such animproved method.

SUMMARY OF THE INVENTION

[0008] The present invention relates to a method for preparingmicroparticles. The present invention allows microparticle products ofvarying polymer molecular weights to be produced using the samemolecular weight starting material. The present invention also allowsmicroparticle products with substantially the same polymer molecularweight to be produced from starting materials of varying molecularweight. In one aspect of the invention, a method of preparingmicroparticles having a selected microparticle polymer molecular weightis provided. The method comprises:

[0009] (a) preparing a first phase, the first phase comprising anucleophilic compound, a polymer having a starting molecular weight, anda solvent for the polymer;

[0010] (b) combining the first phase with a second phase under theinfluence of mixing means to form an emulsion;

[0011] (c) combining the emulsion and an extraction medium, therebyforming microparticles; and

[0012] (d) maintaining the first phase at a hold temperature for a holdperiod prior to step (b), the hold period of sufficient duration toallow the starting molecular weight of the polymer to reduce so that theselected microparticle polymer molecular weight is achieved.

[0013] In a further aspect of the present invention, another method forpreparing microparticles is provided. The method comprises:

[0014] (a) providing a polymer having a starting molecular weight;

[0015] (b) dissolving the polymer and a nucleophilic compound in asolvent to form a first phase;

[0016] (c) combining the first phase with a second phase under theinfluence of mixing means to form an emulsion;

[0017] (d) combining the emulsion and an extraction medium, therebyforming microparticles; and

[0018] (e) maintaining the first phase at a hold temperature for a holdperiod prior to step (c), wherein the hold period is selected so thatthe starting molecular weight reduces so that a selected microparticlepolymer molecular weight is achieved.

[0019] In other aspects of the present invention, the foregoing methodscomprise adding an active agent to the first phase. In yet furtheraspects of the present invention, the foregoing methods comprise addingan inactive agent to the first phase.

[0020] In further aspects of the invention, the hold temperature isincreased, thereby increasing the molecular weight decay of the polymerto reduce the duration of the hold period. The hold temperature can bedecreased, thereby decreasing the molecular weight decay of the polymerto increase the duration of the hold period.

[0021] Other aspects of the present invention include amicroencapsulated active agent and microparticles prepared by themethods of the present invention.

[0022] Features and Advantages

[0023] It is a feature of the present invention that it can be used toprepare microparticles, including microparticles containing an activeagent.

[0024] It is a further feature of the present invention that it allowsthe hold time and temperature of a nucleophilic compound/polymersolution to be modified to achieve a selected polymer molecular weightin the microparticle product.

[0025] An advantage of the present invention is that a selected polymermolecular weight can be achieved in the microparticle product by using avariety of polymers, having varying starting molecular weights, byvarying the hold time of the nucleophilic compound/polymer solution.

[0026] A further advantage of the present invention is thatmicroparticle products of varying polymer molecular weights can beproduced using the same starting polymer, or using a polymer having thesame starting molecular weight.

BRIEF DESCRIPTION OF THE INVENTION

[0027] The present invention is described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements.

[0028]FIG. 1 depicts a graph of molecular weight loss percentage as afunction of solution hold time (hours) at a 1 kg scale;

[0029]FIG. 2 depicts a graph of molecular weight loss percentage as afunction of solution hold time (hours) at a 20 kg scale;

[0030]FIG. 3 depicts a graph of molecular weight (kD) as a function ofsolution hold time (hours) at 15° C., 25° C., and 35° C.; and

[0031]FIG. 4 shows one embodiment of an equipment configuration suitablefor preparing microparticles in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Overview

[0033] The present invention provides an improved method for preparingmicroparticles. The methods of the present invention control the holdtime and temperature of a polymer solution in order to control themolecular weight of the polymer in the finished microparticle product.In this manner, the methods of the present invention advantageouslyallow a selected polymer molecular weight to be achieved from a varietyof starting material molecular weights. Alternatively, microparticleproducts of varying polymer molecular weights can be produced using thesame molecular weight starting material. Thus, a range of products canbe made from the same starting materials, thereby eliminating the needto reformulate the finished product to achieve the desired molecularweight of the polymer in the finished product.

[0034] The polymer solution used in the present invention comprises anucleophilic compound. As used herein, “nucleophilic compound” refers toa compound that promotes by nucleophilic catalysis the ester hydrolysis,such as the polymer scission, that occurs in the biodegradation ofbiodegradable polymers, such as polymers comprising varyinglacotide:glycolide ratios. A nucleophilic compound is a more effectivenucleophile toward an ester group of the polymer than hydroxide ion orwater. Nucleophilic compounds that catalyze the polymer hydrolysisinclude, but are not limited to, amines and carboxylate anions, and canbe “active agents” (defined below) or “inactive agents” that are notactive agents. Examples of nucleophilic compounds that are active agentsinclude, but are not limited to, risperidone, 9-hydroxyrisperidone, andpharmaceutically acceptable salts of the foregoing, naltrexone, andoxybutynin. Examples of nucleophilic compounds that are inactive agentsinclude, but are not limited to, protamine sulfate, spermine, choline,ethanolamine, diethanolamine, and triethanolamine. It should be readilyapparent to be one skilled in the art that the present invention is notlimited to any particular nucleophilic compound, and that the presentinvention encompasses other nucleophilic active agents and nucleophilicinactive agents.

[0035] To ensure clarity of the description that follows, the followingdefinitions are provided. By “microparticles” or “microspheres” is meantparticles that comprise a polymer that serves as a matrix or binder ofthe particle. The microparticle may contain an active agent or othersubstance dispersed or dissolved within the polymeric matrix. Thepolymer is preferably biodegradable and biocompatible. By“biodegradable” is meant a material that should degrade by bodilyprocesses to products readily disposable by the body and should notaccumulate in the body. The products of the biodegradation should alsobe biocompatible with the body. By “biocompatible” is meant not toxic tothe body, is pharmaceutically acceptable, is not carcinogenic, and doesnot significantly induce inflammation in body tissues. As used herein,“body” preferably refers to the human body, but it should be understoodthat body can also refer to a non-human animal body. By “weight %” or “%by weight” is meant parts by weight per total weight of microparticle.For example, 10 wt. % active agent would mean 10 parts active agent byweight and 90 parts polymer by weight. By “controlled releasemicroparticle” or “sustained release microparticle” is meant amicroparticle from which an active agent or other type of substance isreleased as a function of time. By “mass median diameter” is meant thediameter at which half of the distribution (volume percent) has a largerdiameter and half has a smaller diameter.

[0036] By “active agent” is meant an agent, drug, compound, compositionof matter or mixture thereof which provides some pharmacologic, oftenbeneficial, effect. This includes foods, food supplements, nutrients,drugs, vitamins, and other beneficial agents. As used herein, the termsfurther include any physiologically and pharmacologically activesubstance that produces a localized or systemic effect in a patient.Such active agents include antibiotics, antiviral agents, anepileptics,analgesics, anti-asthmatics, anti-inflammatory agents andbronchodilators, and may be inorganic and organic compounds, including,without limitation, drugs which act on the peripheral nerves, adrenergicreceptors, cholinergic receptors, the skeletal muscles, thecardiovascular system, smooth muscles, the blood circulatory system,synoptic sites, neuroeffector junctional sites, endocrine and hormonesystems, the immunological system, the reproductive system, the skeletalsystem, autacoid systems, the alimentary and excretory systems, thehistamine system and the central nervous system. Suitable agents may beselected from, for example, polysaccharides, steroids, hypnotics andsedatives, tranquilizers, anticonvulsants, muscle relaxants,antiParkinson agents, analgesics, anti-inflammatories, musclecontractants, antimicrobials, antimalarials, hormonal agents includingcontraceptives, sympathomimetics, polypeptides and proteins capable ofeliciting physiological effects, diuretics, lipid regulating agents,antiandrogenic agents, leukotriene antagonists, antiparasites,neoplastics, antineoplastics, hypoglycemics, nutritional agents andsupplements, growth supplements, fats, ophthalmics, antienteritisagents, electrolytes and diagnostic agents.

[0037] Method and Examples

[0038] The following examples are provided to explain the invention, andto describe the materials and methods used in carrying out theinvention. The examples are not intended to limit the invention in anymanner.

[0039] Molecular Weight Experiments with Nucleophilic Compounds

EXAMPLE 1

[0040] A series of experiments were conducted at the 1 kg scale thatdemonstrate the relationship between molecular weight of the finishedmicroparticle product, and the duration of a hold period of anucleophilic compound/polymer solution. Microparticles comprisingrisperidone were prepared at the one-kilogram scale. The 1 Kg process(400 grams of active agent and 600 grams of polymer) provides atheoretical drug loading of the microparticles of 40% (400 grams/1000grams×100%).

[0041] A 16.7 wt. % polymer solution was prepared by dissolving 600grams of MEDISORB® 7525 DL polymer (Alkermes, Inc., Blue Ash, Ohio) inethyl acetate. A 24 wt. % drug solution was prepared by dissolving 400grams of risperidone (basic nucleophilic active agent) (JanssenPharmaceutica, Beerse, Belgium) in benzyl alcohol. A nucleophilic activeagent/polymer solution (organic phase) was prepared by mixing the drugsolution into the polymer solution. The active agent/polymer solutionwas maintained at a temperature of 25±5° C. The active agent/polymersolution is held for a hold time of sufficient duration to achieve theselected or desired polymer molecular weight in the finishedmicroparticle product, based on the starting molecular weight of thepolymer. The results of the experiments, showing the effect of hold timeon molecular weight loss, are discussed in more detail below withrespect to Table 1 and FIG. 1.

[0042] The second, continuous phase was prepared by preparing a 30 litersolution of 1% polyvinyl alcohol (PVA), the PVA acting as an emulsifier.To this was added 2086 grams of ethyl acetate to form a 6.5 wt. %solution of ethyl acetate.

[0043] The two phases were combined using a static mixer, such as a ½″Kenics static mixer available from Chemineer, Inc., North Andover, Mass.A total flow rate of 3 L/min generally provides microparticle sizedistributions with a mass median diameter (MMD) in the range of about80-90μ. The ratio of continuous phase to discontinuous phase was 5:1(v/v).

[0044] The quench liquid was 2.5% solution of ethyl acetate andwater-for-injection (WFI) at 5-10° C. The volume of the quench liquid is0.25L per gram of batch size. The quench step was carried out for a timeperiod greater than about 4 hours, with stirring of the microparticlesin the quench tank.

[0045] After completion of the quench step, the microparticles werecollected, de-watered and dried. The temperature was maintained at lessthan about 15° C.

[0046] The microparticles were then re-slurried in a re-slurry tankusing a 25% ethanol solution. The temperature in the re-slurry tank wasin the range of about 0° C to about 15° C. The microparticles were thentransferred back to the quench tank for washing for a time period of atleast 6 hours with another extraction medium (25% ethanol solution) thatwas maintained at preferably 25°±1° C.

[0047] The microparticles were collected, de-watered, and dried. Thetemperature was warmed to greater than about 20° C. but below 40° C.Drying continued for a time period greater than about 16 hours.Twenty-four batches of risperidone microparticles at the 1 kg scale wereprepared using the process described above. Table 1 below shows, foreach batch, the starting molecular weight of the polymer (kD), the finalmolecular weight of the polymer in the finished microparticle product(kD), the percent loss in molecular weight of the polymer, and the holdtime (hours) of the active agent/polymer solution. The molecular weightof the polymer in the finished microparticle product was determined byGPC. TABLE 1 Starting Mw Final Mw Hold time Batch # kD kD % Loss Hours825 230 182 21.0 0.10 708 161 110 32.0 2.08 714 161 133 17.3 0.33 812161 100 37.9 2.40 819 161 102 36.7 2.47 319 131 110 16.2 0.10 331 131115 12.2 0.07 423 131  78 40.7 2.85 506 129 112 13.6 0.07 512 129  8633.7 3.10 520 129  92 29.1 3.07 527 129  95 26.8 2.22 603 129  65 49.46.10 610 129 101 22.0 1.13 617 128  95 26.1 2.20 902 128  85 33.8 1.90908 128  91 29.0 1.18 921 128  99 23.2 0.08 930 128 103 19.4 0.03 915 92  69 24.8 1.82 1021  135 104 23.0 0.03 1028  138 119 13.7 0.45 1110 138 115 16.8 1.28 1215  138 111 19.4 1.50

[0048] The data reported in Table 1 is depicted in the graph shown inFIG. 1. FIG. 1 shows an initial loss in molecular weight ofapproximately 17%, with an additional loss of approximately 5.7% perhour of hold time of the active agent/polymer solution.

EXAMPLE 2

[0049] Additional experiments were conducted at the 20 kg scale thatalso demonstrate the relationship between molecular weight of thefinished microparticle product, and the duration of a hold period of anucleophilic compound/polymer solution. Microparticles comprisingrisperidone were prepared at the twenty-kilogram scale. The 20 Kgprocess (8 kg of active agent and 12 kg of polymer) provides atheoretical drug loading of the microparticles of 40% (8 kg/20 kg×100%).

[0050] A 16.7 wt. % polymer solution was prepared by dissolving 12 kg ofMEDISORB® 7525 DL polymer (Alkermes, Inc., Blue Ash, Ohio) in ethylacetate. A 24 wt. % drug solution was prepared by dissolving 8 kg ofrisperidone (Janssen Pharmaceutica, Beerse, Belgium) in benzyl alcohol.A nucleophilic active agent/polymer solution (organic phase) wasprepared by mixing the drug solution into the polymer solution. Theactive agent/polymer solution was maintained at a temperature of 25±5°C. The active agent/polymer solution is held for a hold time ofsufficient duration to achieve the selected or desired polymer molecularweight in the finished microparticle product, based on the startingmolecular weight of the polymer. The results of the experiments, showingthe effect of hold time on molecular weight loss, are discussed in moredetail below with respect to Table 2 and FIG. 2.

[0051] The second, continuous phase was prepared by preparing a 600liter solution of 1% PVA, the PVA acting as an emulsifier. To this wasadded 42 kg of ethyl acetate to form a 6.5 wt. % solution of ethylacetate. The two phases were combined using a static mixer, such as a1″Kenics static mixer available from Chemineer, Inc., North Andover,Mass.

[0052] The quench liquid was 2.5% solution of ethyl acetate andwater-for-injection (WFI) at 5-10° C. The volume of the quench liquid is0.25L per gram of batch size. The quench step was carried out for a timeperiod greater than about 4 hours, with stirring of the microparticlesin the quench tank.

[0053] After completion of the quench step, the microparticles werecollected, de-watered and dried. The temperature was maintained at lessthan about 15° C.

[0054] The microparticles were then re-slurried in a re-slurry tankusing a 25% ethanol solution. The temperature in the re-slurry tank wasin the range of about 0° C to about 15° C. The microparticles were thentransferred back to the quench tank for washing for a time period of atleast 6 hours with another extraction medium (25% ethanol solution) thatwas maintained at preferably 25°±1° C.

[0055] The microparticles were collected, de-watered, and dried. Thetemperature was warmed to greater than about 20° C. but below 40° C.Drying continued for a time period greater than about 16 hours.

[0056] For batches of risperidone microparticles at the 20 kg scale wereprepared using the process described above. Table 2 below shows, foreach batch, the starting molecular weight of the polymer (kD), the finalmolecular weight of the polymer in the finished microparticle product(kD), the percent loss in molecular weight of the polymer, and the holdtime (hours) of the active agent/polymer solution. The molecular weightof the polymer in the finished microparticle product was determined byGPC. TABLE 2 Starting Mw Final Mw Hold time Batch # kD kD % Loss Hours3308 146 117 20 0.5 4068 145 103 29  1.75 4138 143 111 22 1.0 4208 143110 23 1.0

[0057] The data reported in Table 2 show that from a relatively constantmolecular weight starting material (143 kD, 145 kD, and 146 kD), avariable finished microparticle product molecular weight was achieved byvarying the hold time of the active agent/polymer solution hold time.The data reported in Table 2 is depicted in the graph shown in FIG 2.FIG. 2 shows an initial loss in molecular weight of approximately 16%,with an additional loss of approximately 7.3% per hour of hold time ofthe active agent/polymer solution.

EXAMPLE 3

[0058] The starting molecular weight of the polymer (kD) and the finalmolecular weight of the polymer in a finished microparticle product (kD)was determined for microparticles containing the nucleophilic compoundnaltrexone. The starting polymer lactide:glycolide ratio was 75:25,85:15, and 65:35. The polymers used were MEDISORB® 7525 DL polymer,MEDISORB® 8515 DL polymer and MEDISORB® 6535 DL polymer, all availablefrom Alkermes, Inc., Blue Ash, Ohio.

[0059] The naltrexone base microparticles were produced using aco-solvent extraction process. The theoretical batch size was 15 to 20grams. The polymer was dissolved in ethyl acetate to produce a 16.7% w/wpolymer solution. The naltrexone base anhydrous was dissolved in benzylalcohol to produce a 30.0% w/w solution. In various batches, the amountof drug and polymer used was varied to produce microparticles withdifferent theoretical drug loading ranging from 30% -75%. The ambientpolymer and drug solutions were mixed together until a singlehomogeneous solution (organic phase) was produced. The aqueous phase wasat ambient conditions and contained 1% w/w polyvinyl alcohol and asaturating amount of ethyl acetate. These two solutions were pumped viapositive displacement pumps at a ratio of 3:1 (aqueous:organic) througha ¼″in-line mixer to form an emulsion. The emulsion was transferred to astirring solvent extraction solution consisting of 2.5% w/w of ethylacetate dissolved in distilled water at 5-10° C. and at a volume of 0.5Lof extraction solution per theoretical gram of microparticles. Both thepolymer and drug solvents were extracted into the extraction solutionfrom the emulsion droplets to produce microparticles. The initialextraction process ranged from two to four hours. The microparticleswere collected on a 25 μm sieve and rinsed with a cold (<5° C.) 25% w/wethanol solution. The microparticles were dried cold overnight(approximately 17 hours) using nitrogen. The microparticles were thentransferred to the reslurry solution, which consisted of a vigorouslystirring 25% w/w ethanol solution at 5-10° C. After a short mixing time(five to fifteen minutes), the reslurry solution and the microparticleswere transferred to a stirring 25% w/w ethanol secondary extractionsolution (approximately 25° C. at a volume of 0.2 L of secondaryextraction solution per theoretical gram of microparticles). Themicroparticles stirred for six hours enabling additional solvent removalfrom the microparticles to take place. The microparticles were thencollected on a 25 μm sieve and rinsed with a 25% w/w ethanol solution atambient temperature. These microparticles dried in a hood under ambientconditions overnight (approximately 17 hours), were sieved to removeagglomerated microparticles and then placed into a freezer for storage.

[0060] As shown below in Table 3, three batches of microparticles wereprepared using the 75:25 polymer, two batches for the 85:15 polymer, andfour batches for the 65:35 polymer. For each batch, Table 3 shows thestarting molecular weight of the polymer (kD), and the final molecularweight of the polymer in the finished microparticle products (kD), andthe percent loss in molecular weight of the polymer. The molecularweight of the polymer in the finished microparticle product wasdetermined by GPC. The data in Table 3 provides an example of the lossin molecular weight of the polymer in a finished microparticle productcontaining a nucleophilic compound (naltrexone) for polymers havingvarying lactide:glycolide ratios. TABLE 3 Starting Polymer Starting MwFinal Mw Lactide:glycolide ratio Batch kD kD % Loss 75:25 99-123-004116.2 76.0 34.6 99-123-009 116.2 74.0 36.3 99-123-012 116.2 74.3 36.185:15 99-123-016 109.7 83.7 23.7 99-123-024 109.7 74.9 31.7 65:3599-123-021 102.3 56.3 45.0 99-123-028 102.3 63.4 38.0 99-123-037 102.369.6 32.0 99-123-034 102.3 79.6 22.2

EXAMPLE 4

[0061] Additional experiments were conducted with other polymers thatalso demonstrate the relationship between molecular weight of thefinished microparticle product, and the duration of a hold period of anucleophilic compound/polymer solution. Microparticles comprising otherpolymers having different lactide:glycolide ratios were prepared.Microparticles comprising risperidone using polymers havinglactide:glycolide ratios of 65:35, 85:15, and 100:0 were prepared at the1 Kg scale using the same process described above in Example 1. Thepolymers used were MEDISORB® 6535 DL polymer, MEDISORB® 8515 DL polymer,and MEDISORB® 100 DL polymer, all available from Alkermes, Inc., BlueAsh, Ohio.

[0062] Table 4 below shows, for each polymer, the starting molecularweight of the polymer (kD), the final molecular weight of the polymer inthe finished microparticle product (kD), the percent loss in molecularweight of the polymer, and the hold time (hours) of the activeagent/polymer solution. The molecular weight of the polymer in thefinished microparticle product was determined by GPC. TABLE 4Lactide:glycolide Starting Mw Final Mw Hold time ratio kD kD % Losshours 65:35 105 79 24.8 0.27 85:15 112 96 14.3 0.23 100 dl 105 98  6.70.17

[0063] The data reported in Table 4 show that a microparticle producthaving about the same molecular weight (96 kD and 98 kD) can be preparedfrom two different molecular weight polymers (112 kD and 105 kD,respectively) having two different lactide:glycolide ratios (85:15 and100:0, respectively). The present invention thus advantageously allowsmicroparticle products with the same polymer molecular weight to beproduced using two different starting materials.

EXAMPLE 5

[0064] Additional experiments were conducted that demonstrate themolecular weight loss of polymers in the presence of a nucleophiliccompound (oxybutynin) as a function of time. Tests were conducted usinga 100:0 lactide:glycolide polymer and two 75:25 lactide:glycolidepolymers with differing inherent viscosity. For each test, the followingprotocol was carried out. Weigh about 6 g polymer into an Erlenmeyerflask. Add to the polymer 44 g ethyl acetate, sonicate and shake todissolve the polymer. Weigh 1.5 g oxybutynin base. Stir the polymersolution, and add the drug to the polymer solution. Start the timer asthe drug is added. Sample the drug/polymer solution at 1, 5 and 15minutes, taking about ⅓of the original volume for each aliquot as thesolution stirs. Dispense the aliquot into 250 mL 50:50 H₂O:MeOH, andstir. This mix precipitates the polymer and removes the drug from theprecipitate. Allow precipitated polymer to settle and decant thesupernatant. Wash polymer residue with 100 mL MeOH, stir approximatelyone minute, add up to 250 mL H₂O. Allow polymer to settle again, andrepeat. Residue is then removed from the beaker and placed in ascintillation vial and frozen. Once all samples are collected andfrozen, all samples are placed in a lyophilizer, cooled to −10° C. Thelyophilizer is activated, and once a stable vacuum is achieved, theshelf temperature is raised to 15° C. and held overnight (˜18 hours) toremove residual solvents.

[0065] The results of these experiments are shown in Table 5. For eachexperiment, the starting molecular weight of the polymer is shown, alongwith the polymer molecular weight at 1, 5, and 15 minutes of exposure ofthe polymer to the nucleophilic compound in the drug/polymer solution.As can be seen in Table 5, the longer the exposure or hold time of thedrug/polymer solution, the lower the molecular weight of the polymer.TABLE 5 Starting Polymer Time = Time = Time = Lactide:glycolide Starting1 min 5 min 15 min ratio Mw, kD Mw, kD Mw, kD Mw, kD 100:0  77.1 67.263   60.8 75:25 82.8 56.2 55.1 48.8 75:25 54.1 44.1 42.9 38.4

[0066] Molecular Weight Temperature Experiments

EXAMPLE 6

[0067] Additional experiments were conducted to determine the effect oftemperature on the relationship between molecular weight of the finishedmicroparticle product, and the duration of a hold period of anucleophilic compound/polymer solution. Fifty grams of risperidone(Janssen Pharmaceutica, Beerse, Belgium) were dissolved in 275 g ofbenzyl alcohol to form a drug solution. A polymer solution was formed bydissolving 75 g of MEDISORB® 7525 DL polymer (Alkermes, Inc., Blue Ash,Ohio) in ethyl acetate. The starting molecular weight of the polymer was146 kD. The drug solution and the polymer solution were mixed to form acombined solution. A flask of the combined solution was placed in eachof a 15° C., 25° C., and 35° C. chamber. At periodic time intervals, 10cc of the combined solution was withdrawn from the flask in each chambervia a syringe and needle. The 10 cc sample was then precipitated in abath containing 200 ml methanol at room temperature (approximately 20°C.). The polymeric precipitate was recovered from the methanol bath, andvacuum dried overnight. The dried samples were tested for theirmolecular weight by GPC.

[0068] The results of the experiments are depicted in the graph of FIG.3. As shown in FIG. 3, the molecular weight decay increases astemperature increases. Therefore, by increasing the hold temperature ofthe solution containing the polymer and the nucleophilic compound, themolecular weight decay of the polymer increases, and the duration of thehold period to achieve a particular molecular weight reduction isreduced. Similarly, by decreasing the hold temperature of the solutioncontaining the polymer and the nucleophilic compound, the molecularweight decay of the polymer decreases, and the duration of the holdperiod to achieve a particular molecular weight reduction is increased.For example, the time required to reduce the molecular weight form 130kD to 110 kD is the shortest at 35° C. (about 5 hours) and the longestat 15° C. (about 15 hours).

[0069]FIG. 3 shows an initial increase in polymer molecular weight. Thisphenomenon is most likely occurring because some portion of the polymer,particularly the lower molecular weight fractions is soluble in theextraction medium. Because the analytical measurement of molecularweight is a representation of all the molecular weight fractionspresent, removing (dissolving) the low molecular weight material canincrease the measured molecular weight.

[0070] Methods of Preparing Microparticles

EXAMPLE 7

[0071] As exemplified by the examples discussed above, methods forpreparing microparticles having a selected microparticle polymermolecular weight in accordance with the present invention will now bedescribed in more detail. In one embodiment of the present invention, afirst phase, comprising a nucleophilic compound, a polymer having astarting molecular weight, and a solvent for the polymer, is prepared.In one embodiment of the present invention, the first phase is preparedby dissolving a nucleophilic active agent in a first solvent to form anactive agent solution. The polymer is dissolved in a second solvent toform a polymer solution. The active agent solution and the polymersolution are blended to form the first phase. In a particularlypreferred embodiment, the active agent is selected from the groupconsisting of risperidone, 9-hydroxyrisperidone, and pharmaceuticallyacceptable salts thereof. In such an embodiment, a preferred firstsolvent is benzyl alcohol, and a preferred second solvent is ethylacetate.

[0072] In another embodiment of the present invention, the first phaseis prepared by dissolving the nucleophilic compound and the polymer in asolvent to form a solution. In yet a further embodiment, an active agentis added to the first phase. In a further embodiment, an inactive agentis added to the first phase. It should be understood that the presentinvention is not limited to any particular method or process by whichthe first phase is prepared, and other suitable processes would bereadily apparent to one skilled in the art.

[0073] A second phase is prepared, and combined with the first phaseunder the influence of mixing means to form an emulsion. In a preferredembodiment, a static mixer is used to combine the two phases to form anemulsion. A process for forming an emulsion using a static mixer isdescribed, for example, in U.S. Pat. No. 5,654,008, the entirety ofwhich is incorporated herein by reference. The emulsion is combined withan extraction medium that extracts solvent from the emulsion droplets,thereby hardening them into microparticles.

[0074] Prior to combining the first and second phases, the first phaseis maintained at a hold temperature for a hold period. The hold periodis of sufficient duration to allow the starting molecular weight of thepolymer to reduce to the selected microparticle polymer molecular weightat the hold temperature. Based on the teachings and examples providedherein, and the knowledge of skilled artisans, the determination ofsuitable hold temperatures and hold periods is within the routine skillof skilled artisans and would not require undue experimentation. In apreferred embodiment of the present invention, the starting molecularweight of the polymer reduces by about 10% to about 50% to reach theselected polymer molecular weight. However, it should be understood byone skilled in the art that the present invention is not limited to sucha percentage reduction.

[0075] During the hold period, the first phase may be mixed, stirred, orotherwise agitated. Alternatively, during the hold period, the firstphase may be subjected to no mixing, stirring, or agitation. The holdtemperature is preferably in the range of from about 15° C. to about 35°C., more preferably about 25° C.

[0076] An alternate method for preparing microparticles in accordancewith the present invention will now be described. A polymer having astarting molecular weight and a nucleophilic compound are dissolved in asolvent to form a first phase. An active agent and/or an inactive agentcan be added to the first phase. The first phase is combined with asecond phase under the influence of mixing means to form an emulsion.The emulsion is combined with an extraction medium that extractssolvent, thereby hardening the emulsion droplets into microparticles.Prior to combining the first and second phases, the first phase ismaintained at a hold temperature for a hold period. The hold period isselected so that the starting molecular weight of the polymer reduces toa selected microparticle polymer molecular weight at the holdtemperature. The duration of the hold period can be adjusted by changingthe hold temperature in a manner as described above.

[0077] Microparticles of the Present Invention

[0078] The microparticles prepared by the process of the presentinvention preferably comprise a polymeric binder. Suitable polymericbinder materials include poly(glycolic acid), poly-d,1-lactic acid,poly-1-lactic acid, copolymers of the foregoing, poly(aliphaticcarboxylic acids), copolyoxalates, polycaprolactone, polydioxanone,poly(ortho carbonates), poly(acetals), poly(lactic acid-caprolactone),polyorthoesters, poly(glycolic acid-caprolactone), polyanhydrides, andpolyphosphazines. Poly (d,1-lactic-co-glycolic acid) is commerciallyavailable from Alkermes, Inc. (Blue Ash, Ohio). A suitable productcommercially available from Alkermes, Inc. is a 50:50poly(d,1-lactic-co-glycolic acid) known as MEDISORB® 5050 DL. Thisproduct has a mole percent composition of 50% lactide and 50% glycolide.Other suitable commercially available products are MEDISORB® 6535 DL,7525 DL, 8515 DL and poly(d,1-lactic acid) (100 DL).Poly(lactide-co-glycolides) are also commercially available fromBoehringer Ingelheim (Germany) under its Resomer® mark, e.g., PLGA 50:50(Resomer® RG 502), PLGA 75:25 (Resomer® RG 752) and d,1-PLA (Resomer® RG206), and from Birmingham Polymers (Birmingham, Ala.). These copolymersare available in a wide range of molecular weights and ratios of lacticacid to glycolic acid.

[0079] One type of microparticle suitable for preparation by the presentinvention is a sustained-release microparticle that is biodegradable.However, it should be understood by one skilled in the art that thepresent invention is not limited to biodegradable or other types ofsustained-release microparticles. As would be apparent to one skilled inthe art, the molecular weight of the polymeric binder material forbiodegradable microparticles is of some importance. The molecular weightshould be high enough to permit the formation of satisfactory polymercoatings, i.e., the polymer should be a good film former. However, sincethe properties of the film are also partially dependent on theparticular polymeric binder material being used, it is very difficult tospecify an appropriate molecular weight range for all polymers. Themolecular weight of the polymer is also important from the point of viewof its influence upon the biodegradation rate of the polymer. For adiffusional mechanism of drug release, the polymer should remain intactuntil all of the drug is released from the microparticles and thendegrade. The drug can also be released from the microparticles as thepolymeric binder bioerodes. By an appropriate selection of polymericmaterials a microparticle formulation can be made in which the resultingmicroparticles exhibit both diffusional release and biodegradationrelease properties. This is useful in according multiphasic releasepatterns. A satisfactory starting molecular weight of the polymer is inthe range of 5-500 kD, preferably in the range of from about 50 kD toabout 250 kD. The microparticle polymer molecular weight is preferablyin the range of from about 10 kD to about 185 kD.

[0080] The microparticles prepared in accordance with the presentinvention may include an active agent or other type of substance that isreleased from the microparticles into the host. However, it should beunderstood that the present invention is not limited to preparation ofmicroparticles that contain an active agent. The active agent can be anucleophilic compound. Alternatively, the active agent is not anucleophilic compound and is added to the microparticles during theformation process. Such active agents can include 1,2-benzazoles, moreparticularly, 3-piperidinyl-substituted 1,2-benzisoxazoles and1,2-benzisothiazoles. The most preferred active agents of this kind are3-[2-[4-(6-fluoro-1,2-benzisoxazol-3-yl)-1-piperidinyl]ethyl]-6,7,8,9-tetrahydro-2-methyl-4H--pyrido[1,2-a]pyrimidin-4-one (“risperidone”) and3-[2-[4-(6-fluro-1,2-benzisoxazol-3-yl)-1-piperidinyl]ethyl]-6,7,8,9-tetrahydro-9-hydroxy-2-methyl-4H-pyrido[1,2-a]pyrimidin-4-one(“9-hydroxyrisperidone”) and the pharmaceutically acceptable saltsthereof. Risperidone (which term, as used herein, is intended to includeits pharmaceutically acceptable salts) is most preferred. Risperidonecan be prepared in accordance with the teachings of U.S. Pat. No.4,804,663, the entirety of which is incorporated herein by reference.9-hydroxyrisperidone can be prepared in accordance with the teachings ofU.S. Pat. No. 5,158,952, the entirety of which is incorporated herein byreference.

[0081] Other biologically active agents include non-steroidalantifertility agents; parasympathomimetic agents; psychotherapeuticagents; tranquilizers; decongestants; sedative hypnotics; steroids;sulfonamides; sympathomimetic agents; vaccines; vitamins; antimalarials;anti-migraine agents; anti-Parkinson agents such asL-dopa;anti-spasmodics; anticholinergic agents (e.g. oxybutynin);antitussives; bronchodilators; cardiovascular agents such as coronaryvasodilators and nitroglycerin; alkaloids; analgesics; narcotics such ascodeine, dihydrocodienone, meperidine, morphine and the like;non-narcotics such as salicylates, aspirin, acetaminophen,d-propoxyphene and the like; opioid receptor antagonists, such asnaltrexone and naloxone; antibiotics such as gentamycin, tetracyclineand penicillins; anti-cancer agents; anti-convulsants; anti-emetics;antihistamines; anti-inflammatory agents such as hormonal agents,hydrocortisone, prednisolone, prednisone, non-hormonal agents,allopurinol, indomethacin, phenylbutazone and the like; prostaglandinsand cytotoxic drugs.

[0082] Still other suitable active agents include estrogens,antibacterials; antifungals; antivirals; anticoagulants;anticonvulsants; antidepressants; antihistamines; and immunologicalagents.

[0083] Other examples of suitable biologically active agents includepeptides and proteins, analogs, muteins, and active fragments thereof,such as immunoglobulins, antibodies, cytokines (e.g. lymphokines,monokines, chemokines), blood clotting factors, hemopoietic factors,interleukins (IL-2,IL-3,IL4,IL-6), interferons (β-IFN, α-IFN and γ-IFN),erythropoietin, nucleases, tumor necrosis factor, colony stimulatingfactors (e.g., GCSF, GM-CSF, MCSF), insulin, enzymes (e.g., superoxidedismutase, tissue plasminogen activator), tumor suppressors, bloodproteins, hormones and hormone analogs (e.g., growth hormone,adrenocorticotropic hormone and luteinizing hormone releasing hormone(LHRH)), vaccines (e.g., tumoral, bacterial and viral antigens);somatostatin; antigens; blood coagulation factors; growth factors (e.g.,nerve growth factor, insulin-like growth factor); protein inhibitors,protein antagonists, and protein agonists; nucleic acids, such asantisense molecules; oligonucleotides; and ribozymes. Small molecularweight agents suitable for use in the invention include, antitumoragents such as bleomycin hydrochloride, carboplatin, methotrexate andadriamycin; antipyretic and analgesic agents; antitussives andexpectorants such as ephedrine hydrochloride, methylephedrinehydrochloride, noscapine hydrochloride and codeine phosphate; sedativessuch as chlorpromazine hydrochloride, prochlorperazine hydrochloride andatropine sulfate; muscle relaxants such as tubocurarine chloride;antiepileptics such as sodium phenytoin and ethosuximide; antiulceragents such as metoclopramide; antidepressants such as clomipramine;antiallergic agents such as diphenhydramine; cardiotonics such astheophillol; antiarrhythmic agents such as propranolol hydrochloride;vasodilators such as diltiazem hydrochloride and bamethan sulfate;hypotensive diuretics such as pentolinium and ecarazine hydrochloride;antidiuretic agents such as metformin; anticoagulants such as sodiumcitrate and heparin; hemostatic agents such as thrombin, menadionesodium bisulfite and acetomenaphthone; antituberculous agents such asisoniazide and ethanbutol; hormones such as prednisolone sodiumphosphate and methimazole.

[0084] The microparticles can be mixed by size or by type. However, itshould be understood that the present invention is not limited to theuse of biodegradable or other types of microparticles that contain anactive agent. In one embodiment, the microparticles are mixed in amanner that provides for the delivery of active agent to the host in amultiphasic manner and/or in a manner that provides different activeagents to the host at different times, or a mixture of active agents atthe same time. For example, secondary antibiotics, vaccines, or anydesired active agent, either in microparticle form or in conventional,unencapsulated form can be blended with a primary active agent andprovided to the host.

[0085] Apparatus

[0086] With reference now to FIG. 4, one embodiment is shown of anequipment configuration suitable for use in preparing microparticles inaccordance with the present invention. In a preferred embodiment of thepresent invention, the equipment contained within the dotted lineboundary shown generally at 270 is sterilized using a “steam-in-place”(SIP) process.

[0087] A first phase 201 is provided. First phase 201 is preferably thediscontinuous phase, comprising a polymer dissolved in one or moresolvents, and an active agent. The active agent can be dissolved ordispersed in the same or a different solvent than the solvent(s) inwhich the polymer is dissolved. A second phase 202 is preferably thecontinuous phase, preferably comprising water as the continuousprocessing medium. Preferably, an emulsifying agent such as a surfactantor a hydrophilic colloid is added to the continuous phase to prevent themicrodroplets from agglomerating and to control the size of themicrodroplets in the emulsion. Examples of compounds that can be used assurfactants or hydrophilic colloids include, but are not limited to,poly(vinyl alcohol) (PVA), carboxymethyl cellulose, gelatin, poly(vinylpyrrolidone), Tween 80, Tween 20, and the like. The concentration ofsurfactant or hydrophilic colloid in the continuous phase will be fromabout 0.1% to about 10% by weight based on the continuous processingmedium, depending upon the surfactant, hydrophilic colloid, thediscontinuous phase, and the continuous processing medium used. Apreferred continuous phase is 0.1 to 10 wt. %, more preferably 0.5 to 2wt. %, solution of PVA in water. Although not absolutely necessary, itis preferred to saturate the continuous phase with at least one of thesolvents forming the discontinuous phase.

[0088] First phase 201 and second phase 202 are combined under theinfluence of mixing means to form an emulsion. A preferred type ofmixing means is a static mixer 210. Other mixing means suitable for usewith the present invention include, but are not limited to, devices formechanically agitating the first and second phases, such ashomogenizers, propellers, impellers, stirrers, and the like. Preferably,the discontinuous and continuous phases 201 and 202 are pumped throughstatic mixer 210 to form an emulsion, and into a large volume of quenchliquid, to obtain microparticles containing the active agentencapsulated in the polymeric matrix material. A pump 203 pumps firstphase 201 into static mixer 210, and a pump 204 pumps second phase 202into static mixer 210. An especially preferred method of mixing with astatic mixer in the process of the present invention is disclosed inU.S. Pat. No. 5,654,008, the entirety of which is incorporated herein byreference.

[0089] First and second phases 201 and 202 are mixed in static mixer 210to form an emulsion. The emulsion formed comprises microparticlescontaining active agent encapsulated in the polymeric matrix material.The microparticles are then preferably stirred in a quench or extractiontank 220 containing a quench liquid in order to remove most of thesolvent from the microparticles, resulting in the formation of hardenedmicroparticles. Following the movement of the microparticles from staticmixer 210 and entrance into quench tank 220, the continuous processingmedium is diluted, and much of the solvent in the microparticles isremoved by extraction. In this extractive quench step, themicroparticles can be suspended in the same continuous phase (secondphase 202) used during emulsification, with or without hydrophiliccolloid or surfactant, or in another quench liquid. The quench liquidremoves a significant portion of the solvent from the microparticles,but does not dissolve them. During the extractive quench step, thequench liquid containing dissolved solvent can, optionally, be removedand replaced with fresh quench liquid.

[0090] Upon completion of the quench step in quench tank 220, themicroparticles are transferred by a pump 224 to a device 230 thatfunctions as a microparticle collecting device, de-watering device, anddrying device.

[0091] Device 230 comprises a vibrating sieve or screen. The vibrationcauses smaller particles and liquid to drop through the screen, whilelarger particles are retained. The smaller particles and liquid thatdrop through the screen are removed as waste 235. Device 230 alsofunctions as a vacuum dryer, through the use of a vacuum line 237. Themicroparticles are fluidized by the vibrational energy, and by a smallamount of a dry gas bleed, preferably a dry nitrogen (N₂) bleed 236.

[0092] The dried microparticles are transferred to another extractionmedium to carry out a wash step. The wash step is preferably carried outin quench tank 220, using an extraction medium 222 having a temperaturehigher than the glass transition temperature (T_(g)) of themicroparticles. To carry out the wash step, the microparticles are firstintroduced into a re-slurry tank or other type of vessel 240, as shownby path 231. The temperature of the extraction medium 242 that is usedin vessel 240 is lower than the T_(g) of the microparticles.

[0093] After the wash step is completed in quench tank 220, themicroparticles are again transferred via pump 224 into device 230 forde-watering and final drying. At the completion of final drying, themicroparticles are discharged from device 230 in the manner describedabove into a sifter 250, as shown by path 232. Sifter 250 is used tofractionate the microparticles by size for filling into vials and forbulk in-process testing (e.g., aspect, active agent content, residualsolvents, in vitro release, and particle size distribution).

[0094] Conclusion

[0095] While various embodiments of the present invention have beendescribed above, it should be understood that they have been presentedby way of example only, and not limitation. The present invention is notlimited to the preparation of controlled release microparticles ormicroparticles containing an active agent, nor is it limited to aparticular active agent, polymer or solvent, nor is the presentinvention limited to a particular scale or batch size. Thus, the breadthand scope of the present invention should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

What is claimed is:
 1. A method of preparing microparticles having aselected microparticle polymer molecular weight, comprising: (a)preparing a first phase, the first phase comprising a nucleophiliccompound, a polymer having a starting molecular weight, and a solventfor the polymer; (b) combining the first phase with a second phase toform an emulsion; (c) extracting solvent from the emulsion, therebyforming microparticles; and (d) maintaining the first phase at a holdtemperature for a hold period prior to step (b), the hold period ofsufficient duration to allow the starting molecular weight of thepolymer to reduce so that the selected microparticle polymer molecularweight is achieved.
 2. The method of claim 1 further comprising: (e)increasing the hold temperature, thereby increasing molecular weightdecay of the polymer to reduce a duration of the hold period.
 3. Themethod of claim 1, further comprising: (e) decreasing the holdtemperature, thereby decreasing molecular weight decay of the polymer toincrease a duration of the hold period.
 4. The method of claim 1,wherein the starting molecular weight is in the range of from about 50kD to about 250 kD.
 5. The method of claim 1, wherein the hold period isin the range of from about 0.05 hour to about 6 hours.
 6. The method ofclaim 1, wherein the hold temperature is in the range of from about 15°C. to about 35° C.
 7. The method of claim 1, wherein step (c) comprisescombining the emulsion and an extraction medium.
 8. The method of claim1, wherein the nucleophilic compound is selected from the groupconsisting of risperidone, 9-hydroxyrisperidone, and pharmaceuticallyacceptable salts thereof.
 9. The method of claim 8; wherein the solventcomprises benzyl alcohol and ethyl acetate.
 10. The method of claim 1,wherein the polymer is selected from the group consisting ofpoly(glycolic acid), poly-d,1-lactic acid, poly-1-lactic acid, andcopolymers of the foregoing.
 11. The method of claim 10, wherein thepolymer is poly(d,1-lactide-co-glycolide) having a molar ratio oflactide to glycolide in the range of from about 100:0 to about 50:50.12. The method of claim 1, further comprising: (e) mixing the firstphase during the hold period.
 13. The method of claim 1, wherein theselected microparticle polymer molecular weight is in the range of fromabout 10 kD to about 185.0 kD.
 14. The method of claim 4, wherein theselected microparticle polymer molecular weight is in the range of fromabout 10 kD to about 185.0 kD.
 15. The method of claim 1, wherein thestarting molecular weight reduces by an amount in the range of fromabout 10% to about 50% to reach the selected microparticle polymermolecular weight.
 16. The method of claim 4, wherein the startingmolecular weight reduces by an amount in the range of from about 10% toabout 50% to reach the selected microparticle polymer molecular weight.17. The method of claim 13, wherein the starting molecular weightreduces by an amount in the range of from about 10% to about 50% toreach the selected microparticle polymer molecular weight.
 18. A methodof preparing microparticles, comprising: (a) providing a polymer havinga starting molecular weight; (b) dissolving the polymer and anucleophilic compound in a solvent to form a first phase; (c) combiningthe first phase with a second phase to form an emulsion; (d) extractingsolvent from the emulsion, thereby forming microparticles; and (e)maintaining the first phase at a hold temperature for a hold periodprior to step (c), wherein the hold period is selected so that thestarting molecular weight reduces so that a selected microparticlepolymer molecular weight is achieved.
 19. The method of claim 18,further comprising: (f) increasing the hold temperature, therebyincreasing molecular weight decay of the polymer to reduce a duration ofthe hold period.
 20. The method of claim 18, further comprising: (f)decreasing the hold temperature, thereby decreasing molecular weightdecay of the polymer to increase a duration of the hold period.
 21. Themethod of claim 18, wherein the starting molecular weight is in therange of from about 50 kD to about 250 kD.
 22. The method of claim 18,wherein the hold period is in the range of from about 0.05 hour to about6 hours.
 23. The method of claim 18, wherein the hold temperature is inthe range of from about 15° C to about 35° C.
 24. The method of claim18, wherein step (d) comprises combining the emulsion and an extractionmedium.
 25. The method of claim 18, further comprising: (f) adding anactive agent to the first phase.
 26. The method of claim 18, wherein thenucleophilic compound is selected from the group consisting ofrisperidone, 9-hydroxyrisperidone, and pharmaceutically acceptable saltsthereof.
 27. The method of claim 26, wherein the solvent comprisesbenzyl alcohol and ethyl acetate.
 28. The method of claim 18, whereinthe polymer is selected from the group consisting of poly(glycolicacid), poly-d,1-lactic acid, poly-1-lactic acid, and copolymers of theforegoing.
 29. The method of claim 28, wherein the polymer ispoly(d,1-lactide-co-glycolide) having a molar ratio of lactide toglycolide in the range of from about 100:0 to about 50:50.
 30. Themethod of claim 18, further comprising: (f) mixing the first phaseduring the hold period.
 31. The method of claim 18, wherein the selectedmicroparticle polymer molecular weight is in the range of from about 10kD to about 185.0 kD.
 32. The method of claim 21, wherein the selectedmicroparticle polymer molecular weight is in the range of from about 10kD to about 185.0 kD.
 33. The method of claim 18, wherein the startingmolecular weight reduces by an amount in the range of from about 10% toabout 50% to reach the selected microparticle polymer molecular weight.34. The method of claim 21, wherein the starting molecular weightreduces by an amount in the range of from about 10% to about 50% toreach the selected microparticle polymer molecular weight.
 35. Themethod of claim 31, wherein the starting molecular weight reduces by anamount in the range of from about 10% to about 50% to reach the selectedmicroparticle polymer molecular weight.
 36. Microparticles having aselected microparticle polymer molecular weight prepared by the methodof claim
 1. 37. Microparticles prepared by the method of claim
 18. 38.The method of claim 1, wherein the nucleophilic compound is an activeagent.
 39. The method of claim 1, wherein the nucleophilic compound isan inactive agent.
 40. The method of claim 1, further comprising: (e)adding an active agent to the first phase.
 41. The method of claim 1,further comprising: (e) adding an inactive agent to the first phase. 42.The method of claim 1, wherein the nucleophilic compound is basic. 43.The method of claim 18, wherein the nucleophilic compound is an activeagent.
 44. The method of claim 18, wherein the nucleophilic compound isan inactive agent.
 45. The method of claim 18, further comprising: (f)adding an inactive agent to the first phase.
 46. The method of claim 18,wherein the nucleophilic compound is basic.
 47. The method of claim 1,wherein the nucleophilic compound is naltrexone.
 48. The method of claim1, wherein the nucleophilic compound is oxybutynin.
 49. The method ofclaim 18, wherein the nucleophilic compound is naltrexone.
 50. Themethod of claim 18, wherein the nucleophilic compound is oxybutynin.